Various wiper motors applied to a wiper apparatus to wipe a window glass of a vehicle are known. One of such wiper motors has a swing mechanism built therein, so that an output shaft of the wiper motor swings a wiper arm coupled with a wiper blade to reciprocatingly wipe a surface to be wiped.
In this kind of wiper motor, when an operation of a wiper arm is hindered to exert excessively large load on the output shaft of the wiper motor, the swing mechanism or a deceleration mechanism built therein can be damaged. Thus, the action of such excessively large load must be taken into account in designing the strength of each member constituting these mechanisms.
To avoid this issue, some wiper motor having the swing mechanism is provided with a clutch device in the output shaft thereof. Thus, each member constituting the above-mentioned mechanisms does not require too high strength on the assumption of the action of excessively large load, and the wiper motor is small, lightweight, and inexpensive.
The above-mentioned clutch device has an input disk and a clutch disk to transmit rotational driving force therebetween. The clutch device has a construction to transmit the rotational driving force about an axis of an output shaft by engaging these disks with each other in an axial direction of the output shaft. When overload exceeding a predetermined value acts on the output shaft, the rotational driving force inputted into the input shaft exerts a component force in the axial direction of the output shaft against urging force of an urging member, which is to maintain an engagement between the input disk and the clutch disk. Thus, engaging projected portions from engaging recessed portions are disengaged from each other to idly rotate them relative to each other, to prevent the respective members from being damaged.
The engaging projected portions and the engaging recessed portions of the input disk and the clutch disk exert a component force upward in the axial direction by simple engagement (refer to JP-2505881-Y2), specifically by a torque transmission surface in the rotational direction, that is, by slopes of the engaging surfaces (ascending slopes relative to the direction of action of torque) when the overload acts. The engaging surfaces are in surface contact with each other, to secure the transmission of the rotational driving force.
However, when the input disk and the clutch disk relatively moves in the direction of disengagement against the urging force of the urging member under the action of the overload as described above, the urging force (pressing force) of the urging member gradually increases as the relative displacement stroke of the input disk and the clutch disk increases until they are completely disengaged from each other. Accordingly, operation torque also gradually increases during a period from a start of the overload action to a complete disengagement.
When the engaging surfaces of the engaging projected portions and the engaging surfaces of the engaging recessed portions are brought into surface contact with each other, the frictional force between them is prone to vary. In conjunction with increase in urging force (pressing force) of the urging member, this variation becomes more noticeable, and operation torque becomes unstable.
Thus, magnitude and direction of the above-mentioned component force vary much when repeated engagements and disengagements wear the engaging projected portions and/or the engaging recessed portions. Accordingly, the input disk and the clutch disk cannot be disengaged from each other with high accuracy under the overload exceeding the predetermined value.